SIMBA scientists get up close and personal with sea ice to ground truth remote sensing data

By Peter Rejcek, Antarctic Sun Editor

Posted December 13, 2007

There’s a common expression that the devil is in the details. For scientists who want to describe and predict the processes in a sea ice ecosystem, it’s important to get up close and personal with the ice to take samples and make measurements.

That’s exactly what the SIMBA research teams did in October while drifting with an ice floe for four weeks in the Bellingshausen Sea. The data they gathered will be important to create accurate climate models for predicting factors like gas exchange and heat loss.

Currently, most of today’s sea ice models rely on data from remote sensors, with some theory and inferences from the Arctic thrown in.

Passive Microwaves

Objects at the earth’s surface not only emit infrared radiation, they also emit microwaves at relatively low energy levels. When a sensor detects microwave radiation naturally emitted by the earth, that radiation is called passive microwave. Clouds do not emit much microwave radiation compared to sea ice. Thus, microwaves can penetrate clouds and be used to detect sea ice during the day and night, regardless of cloud cover.

Compared to infrared, microwave emission is not as strongly tied to the temperature of an object. Instead, the object’s physical properties, such as atomic composition and crystalline structure, determine the amount of microwave radiation it emits. The crystalline structure of ice typically emits more microwave energy than the liquid water in the ocean. Thus, sensors that detect passive microwave radiation can easily distinguish sea ice from ocean.

From the National Snow and Ice Data Center

“We have to be quite careful in making these extensions [to Antarctica],” said Steve Ackley, SIMBA principal investigator. “We’re on very shaky ground in making predictions based on the current state of coupled models and the state of knowledge of the processes going on in the sea ice cover in Antarctica.”

There are several different satellite-based studies. Passive microwaves provide information on ice concentration and properties related to the surface, depth of snow and temperature of the snow-ice interface. The latter is particularly important because a high enough temperature allows gases to penetrate the sea ice cover, a key variable for the models.

The problem is that passive microwave readings are not reliable because of seawater flooding at the surface. To improve accuracy, the researchers measured snow-ice boundary temperatures and slush occurrence on the floe. That helps them understand how the temperatures and flooding change over time to compare with concurrent satellite imagery.

Another remote tool is a NASA space-based laser altimeter that provides surface elevation for the Greenland and Antarctic ice caps. It’s also sensitive enough to measure surface elevation of sea ice, which the scientists use to determine ice thickness. Again, how accurate is the satellite data? Ackley said this was the first time the researchers in the field made measurements concurrently with the satellites to correlate the findings.

A third satellite-based system, Radarsat, can provide high-resolution images of ice drift and deformation so the researchers can observe the ice opening and closing. Different types of backscatter also signify different ages of ice. A SIMBA buoy array measured the changes in ice deformation to ground-truth the satellite data.

“We’re pretty happy we were able to correlate our campaigns on the surface with these remote sensing investigations at that time,” Ackley said.